The present invention relates to the field of veterinary medicine, specifically to materials and methods for the immunization of poultry against Escherichia coli infections.
By way of background, Escherichia coli infections in poultry represent a variety of clinical conditions including airsacculitis, pericarditis, and perihepatitis. Collectively, these conditions are a major cause of economic loss to the poultry industry (Gross, 1991). The extent of the economic loss due to diseases caused by E. coli in poultry, however, is often underestimated because a substantial number of birds are condemned at processing plants. For instance, more than 42 million young chickens were condemned due to airsacculitis or septicemia in 1988 (Anon, 1990). In 1989, the number of young chickens condemned was nearly 40 million accounting for approximately 70% of all condemnations.
Both airsacculitis and septicemia are conditions that can be caused by E. coli infections, and not surprisingly, E. coli can be readily isolated from tissues of a large percentage of birds that are condemned in processing plants (Kapur et al., 1991). These isolates fell into several previously described genetic clusters of organisms isolated from clinical cases of arian colibacillosis, swollen head syndrome, and from apparently healthy birds (Whittam and Wilson, 1988; White et al., 1990, 1991), as well as into some clonal groups not previously described. Significantly, none of the isolates recovered from processing plants belonged to clone cluster A1, a majority of whose isolates were recovered from the heart, air sac, or liver, and is thought to be a specialized avian cluster containing pathogenic E. coli (White et al., 1991). In addition, nearly a third of the isolates recovered from birds condemned at processing plants were previously discovered to belong to a clonal group (cluster C) that contains mostly low virulence strains (White et al., 1991). These results, along with serotypic analysis of E. coli isolates from processing plants have indicated that the bacterial strains associated with condemnation at processing plants do not necessarily represent the same population of isolates associated with clinical disease.
As emphasis has previously been placed on routinely collecting and characterizing isolates from clinically ailing birds, there is little information available on the pathogenic attributes of major clones associated with condemned birds. In this study, we assessed the degree of virulence of isolates representing the major clones and clonal groups recovered from birds condemned at processing, and experimentally determined if isolates were able to protect birds against challenge with closely related strains.